The present invention relates to ultrasonic transducers.
Ultrasonic transducers find wide application in industry and in medicine. For example, in certain medical procedures, they are used to produce ultrasonic energy which heats tissue within the body of a living subject. In one such type of procedure, known as ablation, sufficient heat is applied to kill undesired tissue. Typically, this requires heating tissue to a temperature of 60-80° C. Furthermore, it is desirable that tissue be heated rapidly in order to avoid incidental damage to surrounding tissue.
In one common medical application, ultrasonic transducers are utilized in catheters which must pass through small spaces within the body. For example, catheters are often passed through the circulatory system in order to be placed into the heart. It will be appreciated that the ultrasonic transducers on such catheters must be quite small, yet they must be capable of emitting a substantial amount of ultrasonic power in order to be useful.
A typical ultrasonic transducer utilized in a catheter includes an active element in the form of a piezoelectric sleeve. Piezoelectric elements deform physically when subjected to an electric field. Hence, when a sufficiently rapidly varying electrical signal is applied, the piezoelectric sleeve vibrates at ultrasonic frequencies, and ultrasonic energy is radiated. Typically, the piezoelectric sleeve is mounted on a supporting tube, typically made of surgical steel, which provides a lumen for the catheter. The supporting tube must be electrically isolated from the piezoelectric sleeve. When the piezoelectric sleeve vibrates, it produces substantial heat, and excessive heat must not be transmitted to the supporting tube.
In order to provide efficient ultrasonic radiation, a backing medium is usually provided at the inner face of the piezoelectric sleeve. This backing is made of a material which has a substantially different ultrasonic impedance than the piezoelectric material, so that ultrasonic energy impinging upon the interface between the piezoelectric sleeve and the backing medium is reflected outwardly, increasing the total ultrasonic radiation away from the sleeve.
One known backing medium is air. Air backing is achieved by constructing the transducer so that air is in contact with substantially entire inner surface of the active element, providing the necessary reflection of ultrasonic energy. U.S. Pat. Nos. 6,599,288 and 6,607,502, both to Maguire et al., disclose a catheter wherein the ultrasound transducer is mounted onto a catheter shaft without any support structure between the two, i.e., the transducer is suspended about the catheter shaft. This may isolate the transducer from the shaft by providing a layer of air between the two, however, a catheter with such complicated structure is difficult to manufacture and assemble.
Ultrasonic transducers have also been provided with solid backing. For example, the backing could be a brass sleeve inwardly of the piezoelectric sleeve. FIG. 1 is a schematic representation of the internal construction of a known water backed transducer 10. An active element 12 of transducer 10 is a cylindrical sleeve made of a piezoelectric material. Cables 13 provide an electrical actuating signal for transducer 10. For this purpose, conductive regions (not shown) are provided on the outer and inner surfaces of sleeve 12. One conductor of each of cables 13 is connected to the conductive region on the outer surface of sleeve 12, and the other conductor is connected to a backing element 14. Since backing element 14 is made of an electrically conductive material (see below), it will make the necessary electrical contact with the conductive region on the inner surface of sleeve 12.
Backing element 14 is generally cylindrical, made of brass, and is provided inside piezoelectric sleeve 12. Backing element 14, at each axial end, includes a plurality of radially extending protrusions 14a. The diameter of the rear surface of sleeve 12 is greater than the diameter of the opposed surface of backing element 14, so that a cylindrical space 16 is formed there between. Space 16 is filled with water and absorbs heat generated by active element 12. Backing element 14 has an axial bore which receives a supporting tube 18, made of stainless steel. Between tube 18 and backing element 14, there is provided a layer 20 of polyimide insulation. The water in this water backed transducer 10 provides a coolant which reduces heat transmission to the tube. The water back of transducer 10 therefore serves dual functions of providing ultrasonic reflection and limiting heat transfer to supporting tube 18.
Unfortunately, an ultrasonic transducer with a water back tends to produce ultrasonic energy inefficiently and permits an undesirable amount of heat to buildup in the interior of the transducer. Additionally, due to the small size of the catheter, it is difficult to provide high enough flow rate of water to obtain sufficient cooling effect. Typically, this type of transducer will convert electrical energy to ultrasound energy with an efficiency of about 45%, and the interior (center) of the transducer can be heated to a temperature in excess of 300° F. Moreover, a catheter with this complex structure is difficult to manufacture and assemble.